1. Field of the Invention
The present invention relates to a positive type photosensitive composition, the solubility of which, in an aqueous alkaline solution, is enhanced by being exposed to infrared rays. More specifically, the invention relates to a positive type photosensitive composition useful as a recording layer of a planographic printing plate precursor that can be subjected to the so-called direct plate-making method, according to which a printing plate can be directly made up by scanning of an infrared laser on the basis of digital signals received from a computer or the like.
2. Description of the Related Art
Hitherto, various photosensitive compositions have been used for the formation of visible images, or as a material for planographic printing plate precursors. Recently, remarkable developments have taken place in the area of lasers for planographic printing. With particular, regard to both solid lasers and semiconductor lasers having an emission wavelength within a near infrared or infrared wavelength range, it has become easy to obtain lasers which have a high level of output and which are at the same time compact. These lasers are extremely useful as light sources for exposure when printing plates are made up directly on the basis of digital data received from a computer or the like.
Materials which can be used for positive type planographic printing plate precursors applicable for infrared lasers include, as essential components, a binder resin soluble in an aqueous alkaline solution (hereinafter referred to where appropriate as an “alkali-soluble resin”), and an infra red dye which absorbs light to generate heat. When an image is formed in a positive type planographic printing plate precursor, the infra red dye interacts with the binder resin in its unexposed portions (image portions) so as to function as a dissolution inhibitor which can substantially reduce the solubility of the binder resin. On the other hand, in its exposed portions (non-image portions), interaction of the infra red dye with the binder resin is weakened by the heat generated. Consequently, an exposed portion can turn into a state in which it can be dissolved in an alkaline developer, so that an image is formed thereon.
However, insofar as infrared-laser-applicable positive planographic printing plate precursor materials are concerned, differences in the degree of resistance against dissolution in a developer between unexposed portions (image portions) and exposed portions (non-image portions) therein, that is, differences in development latitude have not yet been sufficient under various conditions of use. Thus, problems have occurred insofar that, with changes in conditions of use of materials, materials have tended to be either excessively developed or inadequately developed.
Further, in the image-forming portion, the IR dye or the like only serves as a dissolution inhibitor to the un-exposed portion (the image-forming portion) and the strength of the image-forming portion is not particularly improved by the IR dye in the image formation mechanism. Therefore, attempts are being made to select suitable polymers in order to obtain sufficient strength of the image-forming portion. For example, a technique to use novolak resin as a main binder has been proposed (refer to, for example, Japanese Patent Application Laid-Open (JP-A) No. 10-268512), whereby the printing durability is improved by carrying out burning treatment. However, the recording layer has another problem in that it is inferior in chemical resistance.
On the other hand, from the viewpoint of improving the chemical resistance of the recording layer, an image recording material using a sulfonamide based copolymer having a specific structure as a binder polymer has been proposed (e.g., JP-A No. 11-44956). For this type of polymer, although printing durability is observed if a cleaner is used, the printing durability cannot be improved by burning treatment. If the burning treatment is not carried out (even if the composition of the binder polymer is changed), the printing durability will be insufficient and therefore, adequacy in both chemical resistance and printing durability cannot be obtained at the same time.
Accordingly, attempts are being made to improve the printing durability by forming a predetermined intermediate layer between a support and a recording layer. There is disclosed a case in which an intermediate layer containing a copolymer of a carboxylic acid group-containing monomer and an onium salt-containing monomer is provided on a support which has been subjected to hydrophilization with a silicate (e.g., JP-A No. 2000-108538). According to this method, although printing durability and staining resistance of the non-image-forming portion are improved, chemical resistance is still insufficient. Therefore, there has been a strong desire for an image recording material having excellent chemical resistance and also having printing durability considerably improved by the burning treatment. Hereinafter, the excellent printing durability obtained by the burning treatment may be referred to as “printing durability by burning” in some cases.